1. Field of the Invention
This invention relates to an oscillator that provides a stable reference frequency signal in electronic equipment. Specifically, there is an oscillator that has a trim effect compensation circuit to compensate for error in the frequency versus temperature curve which results when a temperature compensated crystal oscillator is adjusted or trimmed to correct for frequency drift due to aging.
2. Description of Related Art
Temperature Compensated Crystal Oscillators (TCXO""s) which employ thermistor/resistor networks have been in existence for some 45 years. These networks operate on the principle of producing a precisely calibrated, temperature varying correction voltage. When this voltage is applied to the tuning port of a voltage controlled crystal oscillator, it exactly cancels the inherent frequency vs. temperature drift of the quartz crystal and oscillator components.
When the frequency of the crystal drifts over time due to aging effects, it is usually necessary to trim the frequency back to nominal by adjusting some reactance in the oscillator circuit which shifts the operating point of the crystal. This is accomplished either by changing the setting of a mechanically variable capacitor or by adjusting the bias voltage to a variable capacitance varactor diode. This bias voltage is called an electronic frequency adjust (EFA) or control voltage and is a conventionally available terminal for connection to an external voltage on many commercially available temperature compensated crystal oscillators. When the EFA voltage is changed, however, the pulling sensitivity of the circuit is also affected. This means that the exact same temperature compensating voltage from the network will have a slightly different effect on the frequency over temperature, resulting in a rotation of the temperature versus frequency curve which degrades the temperature error. The further the frequency has been adjusted or pulled, the greater the magnitude of the error. This error that results after trimming the frequency back to nominal to compensate for aging has become known as the xe2x80x9cTrim Effect Error.xe2x80x9d
Trim effect error has always been a problem with TCXO""s. Although there are some circuit configurations and techniques which can minimize the effect, there is no way to avoid the problem. For lower tolerance units, this is not a major issue, but as tighter compensation stabilities are required, this trim effect error can become a significant portion of the overall frequency tolerance. FIG. 1 illustrates the typical curve rotation that occurs when a precision TCXO is trimmed to its maximum and minimum frequencies. FIG. 1 shows a graph of percent frequency change in parts per million versus temperature in degrees Celsius for 3 different voltages of electronic frequency adjust, 0, 1.25 and 2.5 volts. The graph shows the frequency shift about the nominal frequency of +10 ppm, 0 and xe2x88x9210 ppm. The graph is normalized about 25 degrees Celsius. With some telecom systems requiring compensation to a level of better than xc2x10.3 ppm to achieve Stratum 3 holdover requirements, the trim effect is a limitation in these types of TCXO applications.
FIG. 2 shows the typical rotation of the frequency vs. temperature characteristic of a standard TCXO that occurs as the center frequency is incrementally trimmed + and xe2x88x92 about 12 ppm from nominal. It can be seen that the slope of the entire curve is progressively skewed as the amount of shift is increased or decreased. A straight line can be fit through each curve resulting in a linear function.
Turning to FIG. 3, a graph of the linearity of trim effect rotation is shown, if a straight line is fit through each of the curves of FIG. 2 and the slopes of the lines are plotted vs. the tuning voltage, (which is proportional to the amount of frequency shift or trim), it can be seen that a very linear, well behaved function is obtained. FIG. 3 is a linear regression of the frequency versus temperature rotation as a function of electronic frequency adjust voltage form 0.5 to 4.5 volts. The y-axis has units of ppm/degree/volt. The x-axis has units of volts of electronic frequency adjust.
A current unmet need exists for an oscillator that can overcome the problems of the prior art due to trim effect error.
It is a feature of the invention to provide an oscillator that has a trim effect compensation circuit.
A further feature of the invention is to provide an oscillator assembly that includes a voltage controlled crystal oscillator that produces a stable reference frequency. The voltage controlled crystal oscillator has a terminal connected to an electronic frequency adjust voltage. A temperature compensation circuit is in communication with the voltage controlled crystal oscillator. The temperature compensation circuit is adapted to provide a temperature compensation voltage to the voltage controlled crystal oscillator. The temperature compensation voltage allows the voltage controlled crystal oscillator to maintain the stable reference frequency as the temperature around the oscillator assembly varies. A trim effect compensation circuit is connected between the temperature compensation circuit and the terminal. The trim effect compensation circuit is adapted to adjust the temperature compensation voltage in response to a change in the electronic frequency adjust voltage.
The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. Further, the abstract is neither intended to define the invention of the application, which is measured by the claims, neither is it intended to be limiting as to the scope of the invention in any way.